The UK news media has been buzzing with reports
of air pollution alerts associated, at least in part, with the long-range
transport of dust from the Sahara. Colleagues from Africa have asked why we in
the UK are worried about the health effects of a relatively rare occurrence of
this long-range dust all the way across Europe, when African countries
experience dust storms of much higher intensity almost daily at some times of
year.

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Alerts are based on the weight of particles - but
size matters too

What the UK air quality alerts, issued by the
Department for Environment, Food & Rural Affairs, don't account for is the
cause behind this large number of microscopic particles in the air.

Instead, the alerts are based on the total weight
of particles that occur in one cubic meter of air at standard temperature and
pressure. But only more sophisticated, and less routine, measurements can
distinguish between particles from different sources, such as local traffic
and/or environmental problems.

So while official alerts have gone out, we have
yet to get a breakdown of the chemical make-up of these particles and where
they are from. When we know this we can then work out what each of the possible
sources is contributing to the overall pollution load.

It is not just chemical composition that matters;
size does too. The current air pollution problem appears, in its worst
hotspots, to be made up of the combination of a) Saharan dust particles with b)
particles made in the atmosphere from gaseous pollutants, along with c)
particles emitted directly from very nearby sources, such as traffic.

Traffic produces millions of nano-particles

Particles are measured on a scale called
particulate matter, or PM. The weight-based measurements split results into
only two classes: PM2.5, which groups together the smallest particles, and
PM10, which totals up all the particles.

Local traffic produces millions of nano-particles
- the smallest particles we are interested in for this part of the story. Let's
imagine that instead of being really, really, small, these nano-particles are
the size of footballs.

On a relative scale, this would make the
particles made in the air mostly about the size of a large car and the Saharan
dust particles would as big as a small detached house. PM2.5 weighs the
footballs and cars, while PM10 weighs everything: footballs, cars, houses and
all.